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assumed (Skeels, 1963), then, with an approximate radius
of 20 km, the top surface would lie between depths of 6
and 8 km and the bottom surface between 10 and 15 km.
These depths were calculated for density contrasts of
-0.20 and - 0.05 g crne3 respectively. This interpreta-
tion is approximate, but it indicates that the source of the
regional feature is a low density body within the upper
crust.
As a preliminary interpretation, a regional anomaly
field (Figure 5) was drawn graphically and subtracted
from the observed Bouguer gravity anomaly field (Figure
4) to produce the residual anomaly map (Figure 6), in
which the sources of the anomalies can be considered to
occur at or close to the surface. There are still areas on
this map, however, where the anomalies depart
systematically from zero, particularly in the southeast cor-
ner, suggesting that the estimate of the regional field
could be further improved.
The main difference between the observed and residual
Bouguer anomaly maps, apart from the general reduction
in the amplitude, is the almost complete removal from the
residual anomaly field of the Bouguer anomaly ridge in
the northeastern part of the area. This is because the ridge
has a long wavelength and has been regarded as a regional
feature. The ENE-trend of the anomalies over the main
part of the dome has been somewhat emphasised, par-
ticularly along the southern margin where a linear con-
tour trend extends from [SJ 04 231 to [SJ 30 371.
The above separation of regional and residual
anomalies is extremely arbitrary, so in the following
discussions reference will be made to both the Bouguer
anomaly and residual anomaly maps (Figures 4 and 6
respectively).
It has been noted above that the lowest Bouguer
anomaly values coincide approximately with the main
outcrop of the oldest rocks in the area, the Llandeilo
sedimentary sequence in the centre of the Berwyn Dome.
This suggests that these rocks, or the underlying Precam-
brian basement, are responsible for the low. On the
original Bouguer anomaly map (Figure 4) the lowest
values occur just to the west of the Llandeilian outcrop,
suggesting additionally that low density rocks are most ex-
tensively developed at depth in this direction.
If the Lower Palaeozoic sedimentary rocks have a
higher density than the Precambrian basement, then it
would be expected that not only would the centre of the
Berwyn Dome be a Bouguer anomaly low but Bouguer
anomaly highs would correspond with the outcrop of the
younger rocks in the area. This tends to be the case for the
Silurian rocks in the northeastern part of the area but
sedimentary rocks of the same age occurring in the axis of
the syncline extending NNE from Lake Vyrnwy, past
Corwen (Figure 4) are associated with a Bouguer anoma-
ly low (cf. Figure 8). Elsewhere on the maps there appears
to be little relationship between Bouguer anomaly values
and stratigraphic horizons. Several reasons can be put
forward to explain this, including changes in the nature of
the Precambrian basement or changes in the thickness
and/or lithology of the Lower Palaeozoic sedimentary
rocks; these alternative interpretations are discussed later.
The Bala-Bryneglwys fault system is a major structural
feature which crosses the northwestern part of the area
(Figure 4) and is thought to have influenced Lower
Palaeozoic and later structures. The nature of the
Talyllyn-Bala-Bryneglwys Fault system has been review-
ed by Basset (1969). The Bala section is not directly con-
nected with the Bryneglwys Fault at surface (Basset, up
cit.) but it could be at depth. The phases, directions and
16
extent of movements along this fracture are not accurately
known but at Bala the post-Ordovician downthrow is of
the order of 610 m to the southeast, and at Wrexham the
pre-Triassic downthrow on the Bryneglwys fault is of the
order of 460 m to the northwest. Later movements of the
system involve downthrows to the northwest. Rast (1969)
suggests that the Bala Fault marks the approximate limit
of Ordovician volcanics whose absence to the southeast is
ascribed to the thickening of Cambrian and Precambrian
rocks (by about 3.7 km) above a magmatic layer.
At Bala, the Bouguer gravity anomaly field shows no
major variation across the fault apart from a local anoma-
ly due to thick superficial deposits. West of Corwen the
Bryneglwys fault system marks the northwest boundary of
the main Bouguer anomaly high, but to the southwest the
gradient zone diverges from the Bryneglwys fault and
follows the line of the SSW-trending, unnamed fault
(Figures 4 and 6). The linear character of the contours is
one of most obvious features in Figure 4 and strongly sug-
gests a deep fault extending to the SSW from near Cor-
wen to about [SJ 03 321. If projected southwards this line
would intersect the Bouguer anomaly low at [SJ 03 281
and continue along the strike of the Caradocian sedimen-
try rocks on the western edge of the dome, suggesting the
possibility of an unmapped major strike fault. The same
line also forms the well defined western boundaries to
high areas of several elements, notably Zn (Figure 28) and
Ni (Figure 32) in stream sediment
Superimposed on the broader scale features described
above are smaller scale variations in the Bouguer anomaly
map whose limited extent suggest a near-surface origin.
Included also in these variations are the effects of observa-
tional and data reduction errors. The anomalies which
appear to have some geological significance are described
below.
a Low Bouguer anomaly values were recorded at sta-
tions situated in valley bottoms where the existence of
thick drift or alluvium seems to provide a likely explana-
tion. The most pronounced anomaly occurs just west of
Corwen [SJ 063 4361, but others occur near Bala Lake
[SH 929 3521 and in the valleys of the River Dee [SH 995
367, SG 039 3881 and Afon Tanat [SJ 155 2471.
b The cause of the roughly circular residual low of - 5
mGal at (SJ 033 284) may be a plug-like acid igneous
body at a shallow depth, although the known quartz-
feldspar-porphyry body at Bryn [SJ 23 261, 20 km to the
east, produces only a small residual anomaly of about 0.5
mGa1. The central part of the anomaly northwest of
Llangynog is based on only one station, in steep terrain,
although the station terrain correction has been accurately
repeated. Geochemical anomalies associated with faults in
the volcanics nearby help to support the intrusive plug
hypothesis.
c Further east the group of anomalies around [SJ 10 301
appear to be related to differential movements of blocks
adjacent to the Craig-y-Glyn, Craig-y-Beri and Rhaeadr
valley faults. The residual anomaly low is centred on a
station over some of the oldest rocks in the area, but
volcanic activity and mineralisation near here suggest
another possible cause of the low (see stream sediment
anomaly group 7).
The aeromagnetic anontab field
Figure 7 shows the aeromagnetic anomalies over the Ber-
wyn area, contoured at intervals of 10 nT above a linear
regional field for the UK, and taken from the
aeromagnetic map of Great Britain (Geological Survey of
Great Britain, 1965).
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